Solid CO₂ in laboratory and in dense molecular clouds

Palumbo M.E. , Baratta G.A. , & Strazzulla G.  Osservatorio Astrofisico di Catania, Italy

Laboratory experiments and theoretical calculations had predicted and ISO observations have confirmed the presence of solid carbon dioxide (CO₂) in icy grain mantles in dense molecular clouds. In fact with the ISO satellite it has been possible to observe both the stretching (about 4.27 microns) and bending (about 15.2 microns) modes of solid CO₂ towards both embedded sources and field stars showing that it is an ubiquitous constituent of icy mantles. However the origin of solid CO₂ is not yet clear. An appreciable abundance of gas phase CO₂ is neither predicted nor observed in the interstellar medium so that its condensation on grains can be neglected. Therefore it has been suggested that it could be formed by grain surface reaction of CO+O, through photolysis of H₂O:CO ice mixtures and/or through ion bombardment of ices by cosmic ray particles. Preliminary comparisons between laboratory spectra of ice mixtures containing CO₂ and ISO spectra have shown that it is not possible to find a single spectrum which fits the CO₂ observed bands and it has been suggested that the observed features are due to the sum of at least two independent components. This agree with the conclusions drawn for the observed solid CO absorption feature which is widely believed to be due to two independent polar and nonpolar components.

We have studied, by infrared absorption spectroscopy, the profile (shape, width and peak position) of the stretching and bending modes of solid CO₂ as formed after ion irradiation of ice mixtures containing C-bearing and O-bearing species such as pure CO and CH₃OH ices, nonpolar mixtures CO:N₂and CO:O₂ and polar mixtures H₂O:CH₃OH, H₂O:CH₄, H₂O:CO, H₂O:NH₃:CH₄, H₂O:N₂:CH₄. Ices to be investigated are accreted on a substrate in thermal contact with a cold finger (10-300 K) placed in a vacuum chamber. Samples can be bombarded during and/or after condensation by energetic ions.

We have found that CO₂ is readily formed after ion irradiation of the above mixtures and that the band profile depends on the initial mixture. We have also studied the effects of warm-up on the profile of the bands. Furthermore we have derived the optical constants, that is the real (n) and imaginary (k) part of the complex refractive index, for pure CO₂ ice at 12 K from transmittance spectra taken at oblique incidence and at different thickness and with the electric vector of the incoming infrared beam parallel (p polarized) and perpendicular (s polarized) to the incidence plane. We have shown that the experimental set-up and the computation procedure used can clearly separate the contribution of n and k to the transmittance spectrum and results obtained are more reliable than others. As an example a reliable set of optical constants is in fact needed when small particles effects are to be taken into account. Finally, we have compared the laboratory spectra with the ISO observation towards NGC7538 IRS1 and other lines of sight available in the literature.